A circuit which limits the direct current flowing through a capacitive load when this load is connected to a DC voltage source is discribed in German Patent No. 40 13 731 C2.
In the case of the circuit which is described in the cited document, an output DC current is always delivered at the output connections when an input DC voltage is applied. As a consequence of this, a current flow through a connected load occurs even at low input DC voltages, so that the connected load is operated in unacceptable voltage ranges.
Another conventional circuit is described in European Patent No. 0 272 514. In this circuit, the maximum possible output DC current rises when the input DC voltage increases from zero volts, proportionally with the input DC voltage.
An object of the present invention is therefore to avoid such unacceptable operating states of the connected load, particularly in the case of excessively low input DC voltages and in addition, to sharply reduce the transition between a phase in which a relatively low maximum possible output DC current can be delivered and a phase in which a significantly higher maximum possible output DC current can be delivered.
The object is achieved in that when the input DC voltage is raised from zero volts to the reference voltage, an output DC current flows through the connected load only when the value of the input DC voltage rises to a value above an undervoltage threshold value which is below the reference voltage.
The desired output response can be implemented through circuit engineering particularly simply in accordance with the present invention. Only a small number of simple components are required to construct the circuit, the dimensioning of the undervoltage zener diode, in particular, defining the undervoltage threshold value. In an exemplary the specific implementation, this undervoltage threshold value is 7.5 volts.
If, when the input DC voltage is raised from zero volts to the reference voltage, the maximum possible output DC current rises suddenly, the input DC voltage exceeding an activation threshold value which is below the reference voltage, this results in greatly shortening the transition between a phase in which a relatively low maximum possible output DC current can be delivered and a phase in which a considerably higher maximum possible output DC current can be delivered.
In accordance with the present invention, the power supply to the connected load is stabilized in that, when the input DC voltage is reduced from a voltage value which is above the activation threshold value to zero volts, the maximum possible output DC current falls suddenly only when a notch threshold value is reached which is below the activation threshold value.
The output response of the circuit in turn can be achieved by an implementation which is particularly simple in circuit engineering terms, in accordance with the present invention.
In order to protect the connected load and, in the same way, to protect the components which are located in the current limiting circuit, it is furthermore provided that the circuit has overvoltage protection, so that no output DC current flows through a connected load as soon as the input DC voltage exceeds an overvoltage threshold value which is above the reference voltage.
The described output response of the circuit can be achieved in turn by a simple implementation in circuit engineering terms, the dimensioning of an overvoltage zener diode, in particular, defining the overvoltage threshold value. In an exemplary implementation, this overvoltage threshold value is 36 volts.